This invention relates to a wafer processing machine. It has particular application to a machine of the kind in which a silicon wafer is held on the top of a rotatable chuck and is rotated under precise speed control at two widely disparately speed ranges in the course of performing processing operations on the wafer periphery and upper surfaces. For example, in one particular application of the present invention a silicon wafer is rotated at low speed during a grinding operation while a grinding wheel engages the periphery of the wafer to eliminate sharp corners of the edge profile of the wafer. The wafer is then rotated at high speed during a subsequent rinsing and drying operation.
Silicon wafers are used as the base material in the manufacture of semiconductor devices.
The wafers are cut from bar stock; and before integrated circuits or chips are fabricated on the wafer (by well-known techniques, such as depositing, etching, and the like) the entire peripheral edge of the wafer is contoured to a pre-selected configuration and finish by grinding the edge with a grinding wheel.
In the edge-grinding operation, the wafer is held on top of a chuck with the periphery of the wafer extending outwardly from the chuck for engagement by a grinding wheel. The chuck is rotated at slow speed while the grinding wheel grinds the edge to the proper contour.
The wafers range in diametral size from 2 inch to 5 inch, usually in one inch increments. The purpose of grinding is to eliminate the sharp corners of the edge profile of the wafer. The usual profile obtained is that of a semicircular shape (as viewed in elevation through a cross section of the edge). Rounding off the sharp edges to such a semicircular shape renders the wafer less susceptible to chipping damage in subsequent automatic machine handling. Other benefits obtained from this rounded edge include reduced photo resist edge beading and reduced epitaxial edge crown.
After the edge of the wafer has been ground to the proper contour, the wafer is rinsed with a liquid to remove particles produced by the grinding operation and is then dried.
The drying operation is performed by rotating the disc at relatively high speed to remove the washing liquid by centrifugal action and to provide rapid drying which permits increased production of the wafers.
Other processing operations may be performed on the wafer. For example, the upper surface may be mechanically cleaned by rotating the wafer at low speed while a brush is engaged with the upper surface. A coating operation may also be performed on the upper surface while the wafer is rotated at low speed.
The silicon wafer is quite thin and is relatively fragile, and it is very important to maintain good contact at all times between the grinding wheel and the periphery of the wafer. Slack or rebounding in the drive can result in skipping or chattering of the engagement of the grinding wheel with the periphery of the wafer during the contouring of the periphery of the wafer. Any such skipping or chattering can result in improper grinding or even breakage of the disc and can produce an unacceptable amount of rejects.
The chuck used for wafer processing machines of this kind is often a vacuum chuck which grips the undersurface of the wafer. The drive system must provide a steady, precise drive to prevent the wafer from being shifted off-center (or pushed off the chuck entirely) by the forces produced in the grinding, cleaning or other processing operations.